1. Field of the Invention
Civilizations throughout history have caused pollution of air and water due to agriculture, industry and domestic activities. Such pollution, if significant and if left untreated, can result in serious harm to freshwater and marine ecosystems. A recent example involves the pollution and resulting ecological impairment of the Chesapeake Bay, contiguous estuaries and watershed streams and reservoirs.
Because of these threats to the environment, various wastewater treatment systems have been developed to reduce, remove, and/or transform pollutants, thereby reducing their intrinsic capacity to do harm to the environment. In more recent times, a new paradigm has emerged related to development of environmentally sustainable technologies. These new technologies are being developed to cope with the continued depletion of resources and the ever rising costs of energy. For example, natural treatment systems such as constructed wetlands are being developed, refined and evaluated because of their low capital and operating costs, and because of the robust and sustainable nature of the technology
Properly designed and operated surface- and subsurface-flow constructed wetlands can provide excellent and cost-effective removal of biological oxygen demand (BOD.sub.5) suspended solids (SS), pathogenic bacteria, and other wastewater constituents from domestic, industrial and agricultural wastewaters. Several international symposia have been convened in the past decade regarding the utility of constructed wetlands for treating various types of wastewater. D. A. Hammer (eds.), Constructed Wetlands for Wastewater Treatment: Municipal, Industrial, and Agricultural, Lewis Publishers, Chelsea, Mich., 1989!; Cooper, et al. (eds.), Constructed Wetlands in Water Pollution Control, Pergamon Press, New York, N.Y., 1990!; and G. A. Moshiri (ed.) Constructed Wetlands for Water Quality Improvement, Lewis Publishers, Boca Raton, Fla., 1993!.
Wetlands, if designed and operated properly can provide a great diversity of physical, chemical and biological environments for degrading complex and oftentimes toxic pollutants. In many instances, sequential aerobic and anaerobic environments are required for degrading these complex organic compounds Zitomer, et al., "Sequential Environments for Enhanced Biotransformation of Aqueous Contaminants," Environ. Sci. Tech., 27:227-244 1993!.
However, due to high levels of biological and chemical oxygen demand in the rootzone and interstitial water of wetlands, sufficient dissolved oxygen is not always available for aerobic oxidation of many toxic compounds. For example, adequate levels of dissolved oxygen (&gt;1.0 ppm), are usually not available for optimizing the biological transformation of ammonia to nitrate, referred to as nitrification. In a recent article it was concluded that effective nitrogen (ammonia) removal will require longer hydraulic retention times and/or larger (costlier) wetlands, due to low oxygen availability Reed, et al., "Constructed Wetland Design--the First Generation," Water Environment Research, (64): 776-781, 1992!. Furthermore, under conditions of elevated temperature and pH, ammonium (NH4+) is chemically transformed to its very toxic unionized form (NH3) and can adversely affect the aquatic biota of receiving streams. Similarly, certain dissolved metals, such as the reduced form of manganese, are toxic and difficult to remove from domestic and industrial wastewater streams. By providing coupled aerobic and anaerobic wetland environments, it is possible to remove metals either as metal-sulfides or metal-oxides, respectively.
The instant invention relates to a wastewater treatment system whereby individually paired or sequentially paired subsurface flow wetland cells are operated in such a way as to significantly increase the biomass and diversity of aerobic, facultative anaerobic and anaerobic biofilm organisms, thereby increasing the aerobic and anaerobic environments required for aerobic-anaerobic treatment processes. The action or technique used to facilitate the formation of these recurrent environments is herein referred to as recurrent reciprocation. Examples are provided infra to illustrate the mechanisms which enhance the broad utility of the instant invention and how the technology may be used with respect to domestic, industrial and agricultural wastewaters; the preferred embodiments of the present invention further address these problems.
2. Description of the Prior Art
There appears to be no prior art available which teaches constructed wetland wastewater applications utilizing the design or concept of recurrent reciprocation to affect oxygen transfer and the formation of recurrent and sequential aerobic and anaerobic treatment environments. For instance, in U.S. Pat. No. 681,884, Sep. 3, 1901, Monjeau, there is illustrated a vegetated trickling filter in which polluted water is irrigated unto a single cell that contains a perforated tray with vegetation followed by various-sized sub-terranean substrates to affect removal of sediments and nutrients. There is a provision for several trays of vegetation to be available for interchanging on a regular basis "so that the vegetation may not be destroyed." The design includes an "eccentrically-pivoted flap-valve" near the bottom of the treatment cell to periodically void water from the overlying sediments. However there is no provision for paired cells or recurrent reciprocation.
U.S. Pat. No. 3,770,623, Nov. 6, 1973, Seidel, teaches a horizontal flow subsurface flow wetland system (no recurrent reciprocation), in which various aquatic reeds are planted to purify polluted water. Seidel's design incorporates coarse substrate (gravel), on the bottom of the treatment bed and fine substrate (sand), on the top of the treatment bed for the expressed purposes of enhancing hydraulic conductivity and filtering of fine particulate matter, respectively. No mention is made of using fine-grained materials such as sand, to increase surface area for microbial colonization or enhance gaseous diffusion. The filtration beds (cells), as depicted in Seidel, supra, are in pairs to facilitate removing a single cell from service so that the fine sand layer can be cleaned and rejuvenated periodically since the fine sand layers are prone to plugging, especially under continuous anaerobic conditions. Suspended and dissolved material are removed in separate beds using different types of vegetation.
U.S. Pat. No. 4,331,538, May 25, 1982, Kickuth, discloses a horizontal flow wetland treatment bed supplemented with a soil matrix including iron and/or aluminum, and planted with aquatic plants to facilitate the removal of phosphorus from aqueous liquids such as sewage. Kickuth refers to formation of aerobic and anaerobic micro-environments, the aerobic micro-environments of which are reputedly due to the transport of oxygen from aquatic plant species into the root zone of the treatment bed. Although this is a mechanism for moving oxygen into the rootzone, subsequent mass balance research has shown this mechanism to be insufficient for meeting the respiration and oxygen demand of the root zone environment Brix et al., "Soil Oxygenation in Constructed Reed Beds: The Role of Macrophyte and Soil-atmosphere Interface Oxygen Transport," Constructed Wetlands in Water Pollution Control, 1990, pp. 53-67!.
In U.S. Pat. No. 5,174,897, Dec. 29, 1992, Wengrzvnek, describes a unique design of a surface-flow constructed wetland to control non-point source pollution, but all applications relate to surface flow wetlands with no mention of reciprocating mechanisms, or of subsurface flow modifications.
In U.S. Pat. No. 5,337,516, Aug. 16, 1994, Hondulas, teaches an invention consisting of a wastewater treatment basin and a number of emergent wetland plants in floating containers adapted to float in the wastewater basin such that the root systems of the floating plants treat the wastewater. However, there is no mention of subsurface flow or reciprocating mechanisms.
In the scientific literature, various wastewater applications have been evaluated with respect to enhancing oxygenation of water within constructed wetlands. Aeration of surface- or subsurface-flow wetlands with conventional surface aeration equipment is usually not practical, as the zone of influence is usually restricted to a small area proximate to the aeration device. Fine-bubble aeration is also not practical under conditions of shallow water (30 to 60 cm water depth is typical of surface- and subsurface-flow constructed wetlands), and ambient pressure (one atmosphere), because oxygen transfer (g/m.sup.2 /unit time) is limited by low hydrostatic and atmospheric pressure, and therefore not sufficient for maintaining oxygen concentrations required for most aerobic processes. This is especially true in wastewater treatment situations were biological oxygen demand is high due to biological respiration.
A unique class of subsurface flow wetlands, referred to as vertical-flow subsurface-flow wetlands, have also been designed based on enhancing oxygen transfer Watson, et al., "Pilot-Scale Nitrification Studies using Vertical Flow and Shallow Horizontal Flow Constructed Wetland Cells," in G. A. Moshiri (ed.), Constructed Wetlands for Water Quality Improvement, CRC Press, Inc., Boca Raton, Fla., 1993 pp. 301-315!. Vertical flow systems incorporate an unsaturated zone (periodically voided of water), and work well, especially for small back-yard applications, but are prone to plugging due to microbial growths occurring between the fine sand particles. This is especially problematic in situations where the environment becomes anaerobic, thereby promoting the growth of slime bacteria, which exacerbate the problem of plugging.
Vertical flow designs require frequent irrigation of water onto the surface of a complex graded substrate (fine sand on top, followed by medium sand, followed by pea gravel), and are designed for sequential batch loading (no recurrent or reciprocating component). Reciprocating systems, on the other hand, are designed for batch and/or flow-through of wastewater, with water being pumped back and forth between contiguous basins on a recurrent schedule. This recurrent feature allows fixed-film microbial fauna and associated back-fill material(substrate), to be sequentially exposed to aerobic and anaerobic environments on a controlled basis. It is this unique combination of recurrent movement and controlled sequential exposure of wastewater and substrate to alternating environments that enables the subsequent development of unique microbial consortia and the enhancement of wastewater treatment.
Other gravel-based designs require expensive high-pressure electrical pumps and irrigation systems to spray irrigate the water over raised gravel beds Askew, et al., "Constructed Wetland Recirculating Gravel Filter System: Full-Scale Demonstration and Testing," in On-Site Wastewater Treatment, Proceedings of the Seventh International Symposium on Individual and Small Community Sewage Systems, American Society of Agricultural Engineers, St. Joseph, Mich., 1994, pp. 85-94!. Although vertical flow wetlands and raised gravel-bed wetlands can function efficiently, they are costly to install, operate, and maintain for the reasons cited above, and do not have the unique feature of recurrent reciprocation that readily enhances wastewater treatment.
A team of researchers at the U.S. Virgin Islands have developed a recirculating reciprocating system (no recurrent movement of water between contiguous cells), for fish culture applications. However, the system is managed for aerobic processes, e.g., nitrification J. E. Rakocy, "A Recirculating System for Tilapia Culture and Vegetable Hydroponics," in R. O. Smitherman and D. Tave (eds.), Proceedings Auburn Symposium on Fisheries and Aquaculture, Brown Printing Company, Montgomery, Ala., 1990, pp. 103-114!. In the aforementioned arrangement, a pretreatment system was installed to remove organic carbon (and associated biochemical oxygen demand), in a clarifier prior to water reaching the reciprocating system. This particular design, due to the carbon-removal pretreatment, resulted in formation of acidity from the nitrification process which eventually decreased the buffering capacity of the alkalinity system. To control acidity, a basic solution (potassium hydroxide), had to be added to the system on a routine basis to restore alkalinity. Furthermore, since the reciprocating system was operated in an aerobic mode, little denitrification occurred and as a result, nitrate accumulated to concentrations in excess of 100 ppm. As will be seen later, the instant invention includes natural waste treatment processes, such as calcite dissolution, sulfate reduction, denitrification and methanogenesis, which can restore alkalinity and control acidity, thus obviating the need for exogenous additions of alkalinity-producing chemicals.
It should also be noted that many wastewater treatment constructed wetlands are installed in pairs, or in multiples, both for reasons of enhanced treatment, and for reasons related to maintenance. For example, if there are problems related to bacterial clogging (see Seidel, '623, supra), a single wetland cell can be taken out of service for repair, while the other cell(s) stays "on-line." This ensures that at least one cell will always be available for wastewater treatment functions. This multiple cell horizontal-flow design concept of the prior art now fortuitously allows a relatively simple retrofit of the reciprocation process, thereby providing opportunities to upgrade existing subsurface flow wetlands which have failed and are in violation of discharge permit criteria.
Based on the documented understanding that subsurface-flow constructed wetlands are generally deficient in dissolved oxygen, efforts were undertaken in 1993 at the Tennessee Valley Authority (TVA), to evaluate the impact of reciprocation on oxygen transfer. A series of studies were initiated at the TVA Environmental Research Center facility in Muscle Shoals to determine the efficacy of a reciprocation process, whereby adjacent gravel-backfilled cells were alternately drained and filled with water, on a recurrent basis, to enhance transfer of atmospheric oxygen to the bulk water via diffusion. These studies were performed in oxygen deficient and non-biological systems to observe oxygen transfer rates in the absence of biological respiration. The initial studies were conducted in non-biological systems so that respiration (uptake of oxygen via biotic organisms), would not interfere with the measurement of oxygen transfer. Reciprocation, as practiced in these initial studies, was accomplished by sequentially moving water from one wetland cell to a contiguous cell on a batch loaded (no flow-through), and recurrent basis. Movement of water to initiate reciprocation can be accomplished with gravity, electrical and/or solar pumps, and/or u-tube air-lift principals. With u-tube air-lift applications, enhanced oxygenation is brought about by both mechanical means (u-tube air-lift and associated fine-bubble aeration), and by increased oxygen diffusion due to exposure of wetted surfaces of the abiotic rock (substrate), during the draw-down phase. Results of this preliminary study, detailing aspects of reoxygenation of water in deoxygenated and non-biological systems, were published in a regional symposium L. L. Behrends, et al., "Oxygen Diffusion Rates in Reciprocating Rock Biofilters: Potential Applications for Subsurface Flow Constructed Wetlands," in Proceedings Subsurface Flow Constructed Wetlands Conference, University of Texas at El Paso, Aug. 16-17, 1993!.
Another study was also conducted at the same TVA facility for purposes of evaluating the impact of reciprocation on the removal of metals, specifically manganese, from simulated acid mine drainage water, Sikora, et al., "Manganese and Trace Metal Removal in Successive Anaerobic and Aerobic Wetland Environments," American Power Conference, Chicago, Ill., Apr. 18-20, 1995, sponsored by Illinois Institute of Technology!. In this study, recurrent reciprocation enhanced gas transfer via abiotic mechanisms in accordance with physical gas laws, such that oxygen concentrations and redox potential increased, while carbon dioxide concentrations decreased leading to a significant increase in pH. The combination of increased redox potential and increased pH, governed by the aforementioned gas laws, resulted in the putative abiotic removal of manganese. Abiotic processes are thought to be the major removal mechanism regulating removal of manganese (as oxides), in aerobic subsurface-flow wetlands. Other studies have also been concluded that support the abiotic removal process. McMillen et al., "Constructed Vertical Flow Aerated Wetlands: Manganese Removal from Acid Mine Drainage," Proceedincrs American Water resources Association Annual Summer Symposium, Jackson Hole, WY, Jun. 26-29, 1994, studied the use of unsaturated vertical flow wetlands to provide enough oxygen for manganese oxidation and precipitation and found that manganese could be removed at initial concentrations ranging from 0.5 to 60 mg/l. When a biocide was added to their study, manganese removal remained high, which indicated that abiotic catalysis of manganese oxidation was the controlling factor for manganese removal.
Subsequent to the above mentioned studies, other studies (unpublished and the basis for this patent application) were undertaken in complex biological systems to better understand the relationship between oxygen dynamics, oxygen concentration and redox potential, on rates of nitrification, denitrification, aquatic plant growth, and the removal of carbon (as measured by reduction of COD, the chemical oxygen demand of the wastewater), total nitrogen, orthophosphate and fecal coliform bacteria. It was only after conducting these additional and unpublished studies that it became apparent that it was not the reaeration of the water per se, as was concluded in the aforementioned published articles, that was important to the above noted biological processes, but rather the recurrent exposure of the complex and diverse hiofilm to atmospheric oxygen on a frequent basis, followed by the subsequent inundation of the biofilm by anoxic water.